1. Field of the Invention
The preferred embodiment relates generally to an electric power supply system and method for charging an electric storage battery of a hybrid electric vehicle.
2. Description of the Prior Art
The powertrain of a hybrid electric vehicle includes multiple power sources, an internal combustion engine (ICE), principally used when highway speeds are sustained; an electric motor for charging electric storage batteries; and a traction motor for launching the vehicle, i.e., for accelerating the vehicle from a stopped condition.
Generally, when using sealed lead acid batteries, a constant voltage charge method is the preferred means for charging battery cells. Under a constant voltage regime currents are limited by the internal resistance of the battery. When the battery is in a low state of charge and the internal resistance is low, in-rush currents can be very large and energy can be restored to the battery very quickly. As the battery becomes charged, it reaches a transition point where a sudden rise in internal resistance occurs and, under constant voltage, the battery will accept less and less current. This self-regulating effect prevents overcharging of the battery, leading to longer battery lifetimes. Such constant voltage charge regimes are conducted at an equalization voltage, which is the recommended voltage at which the batteries can be maintained at a high state of charge (SOC).
After this transition a low current equalization charge is required to return the complete capacity of the battery and to ensure that the individual cells within the battery are brought to an approximately equal charge state.
Most of the charging and discharging of the battery is done in a bulk region below the transition point.
Conventional hybrid electric vehicle powertrains perform battery equalization using an electric motor driven by the ICE as a battery charger. Control of the equalization process is difficult due to the mismatch in power ratings of the electric motor-ICE combination and the battery power required to perform a precise overcharge/equalization of the storage battery.
There is a need in the industry for a system and method that provides an equalization charge to a storage battery in a hybrid electric system using a power source other than those onboard the vehicle. There is a need for a system and method that uses customized battery power limits in a hybrid electric powertrain system to improve battery equalization for a vehicle with a plug-in charger.